bases
DESCRIPTION
Bases. Graphics Source: Wikipedia. Acid/Base Definitions. Arrhenius Model Acids produce hydrogen ions in aqueous solutions Bases produce hydroxide ions in aqueous solutions Bronsted-Lowry Model Acids are proton donors Bases are proton acceptors Lewis Acid Model - PowerPoint PPT PresentationTRANSCRIPT
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Bases
Sodium hydroxide
Preferred IUPAC nameSodium hydroxide
Systematic nameSodium oxidanide
Other namesCaustic soda
Lye
Graphics Source: Wikipedia
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Acid/Base Definitions Arrhenius Model
Acids produce hydrogen ions in aqueous solutions
Bases produce hydroxide ions in aqueous solutions
Bronsted-Lowry Model Acids are proton donors Bases are proton acceptors
Lewis Acid Model Acids are electron pair acceptors Bases are electron pair donors
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Dissociation of Strong Bases
Strong bases are metallic hydroxidesGroup I hydroxides (NaOH, KOH) are
very solubleGroup II hydroxides (Ca, Ba, Mg, Sr)
are less solublepH of strong bases is calculated
directly from the concentration of the base in solution
MOH(s) M+(aq) + OH-(aq)
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Reaction of Weak Bases with Water
The base reacts with water, producing its conjugate acid and hydroxide ion:
CH3NH2 + H2O CH3NH3+ + OH- Kb = 4.38 x 10-4
4 3 3
3 2
[ ][ ]4.38 10
[ ]b
CH NH OHK x
CH NH
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Kb for Some Common Weak Bases
Base FormulaConjugat
e AcidKb
Ammonia NH3 NH4+ 1.8 x 10-5
Methylamine CH3NH2 CH3NH3+ 4.38 x 10-4
Ethylamine C2H5NH2 C2H5NH3+ 5.6 x 10-4
Diethylamine (C2H5)2NH (C2H5)2NH2+ 1.3 x 10-3
Triethylamine (C2H5)3N (C2H5)3NH+ 4.0 x 10-4
Hydroxylamine HONH2 HONH3+
1.1 x 10-8
Hydrazine H2NNH2 H2NNH3+
3.0 x 10-6
Aniline C6H5NH2 C6H5NH3+
3.8 x 10-10
Pyridine C5H5N C5H5NH+ 1.7 x 10-9
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Reaction of Weak Bases with Water
The generic reaction for a base reacting with water, producing its conjugate acid and hydroxide ion:
B + H2O BH+ + OH-
[ ][ ]
[ ]b
BH OHK
B
(Yes, all weak bases do this – DO NOTendeavor to make this complicated!)
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A Weak Base Equilibrium Problem
What is the pH of a 0.50 M solution of ammonia, NH3, Kb = 1.8 x 10-5 ?
Step #1: Write the equation for the reaction
NH3 + H2O NH4+ + OH-
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A Weak Base Equilibrium Problem
What is the pH of a 0.50 M solution of ammonia, NH3, Kb = 1.8 x 10-5 ?
Step #2: ICE it!
I
C
E
0.50 0 0
- x +x +x
0.50 - x xx
NH3 + H2O NH4+ + OH-
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A Weak Base Equilibrium Problem
Step #3: Set up the law of mass action
0.50 - x xxE
)50.0()50.0(
))((108.1
25 x
x
xxx
What is the pH of a 0.50 M solution of ammonia, NH3, Kb = 1.8 x 10-5 ?
NH3 + H2O NH4+ + OH-
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A Weak Base Equilibrium Problem
Step #4: Solve for x, which is also [OH-]
0.50 - x xxE
)50.0(108.1
25 x
x [OH-] = 3.0 x 10-3 M
NH3 + H2O NH4+ + OH-
What is the pH of a 0.50 M solution of ammonia, NH3, Kb = 1.8 x 10-5 ?
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A Weak Base Equilibrium Problem
52.4)100.3log( 5 xpOH
Step #5: Convert [OH-] to pH
0.50 - x xxE
What is the pH of a 0.50 M solution of ammonia, NH3, Kb = 1.8 x 10-5 ?
NH3 + H2O NH4+ + OH-
48.900.14 pOHpH
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• Soft cover book Pg. 291-294
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• Text Pg. 713 do question 16.75• Then you can practice for hw 16.76
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• For conjugate acid base pairs only• Ka x Kb = Kw
• pKa = -logKa
• pKb = -logKb
• pKa + pKb = 14
Now do question 16.78 on Pg. 713
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Hydrolysis of Salts
• Salt of strong acid and strong base – neutral
• Salt of weak acid and strong base – basic• Salt of strong base and weak acid – acidic• In general Ka>Kb acidic• Kb>Ka basic• Soft cover book Pg. 387
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• Text Pg. 697• Text pg. 698 sample 16.18 and 16.19
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Reactions of Anions with Water
• Anions are bases.• As such, they can react with water in a
hydrolysis reaction to form OH− and the conjugate acid:
X−(aq) + H2O(l) HX(aq) + OH−(aq)
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Reactions of Cations with Water
• Cations with acidic protons (like NH4
+) will lower the pH of a solution.
• Most metal cations that are hydrated in solution also lower the pH of the solution.
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Reactions of Cations with Water
• Attraction between nonbonding electrons on oxygen and the metal causes a shift of the electron density in water.
• This makes the O-H bond more polar and the water more acidic.
• Greater charge and smaller size make a cation more acidic.
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Effect of Cations and Anions
1. An anion that is the conjugate base of a strong acid will not affect the pH.
2. An anion that is the conjugate base of a weak acid will increase the pH.
3. A cation that is the conjugate acid of a weak base will decrease the pH.
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Effect of Cations and Anions
4. Cations of the strong Arrhenius bases will not affect the pH.
5. Other metal ions will cause a decrease in pH.
6. When a solution contains both the conjugate base of a weak acid and the conjugate acid of a weak base, the affect on pH depends on the Ka and Kb values.
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Factors Affecting Acid Strength
• The more polar the H-X bond and/or the weaker the H-X bond, the more acidic the compound.
• Acidity increases from left to right across a row and from top to bottom down a group.
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Factors Affecting Acid Strength
In oxyacids, in which an OH is bonded to another atom, Y, the more electronegative Y is, the more acidic the acid.
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Factors Affecting Acid Strength
For a series of oxyacids, acidity increases with the number of oxygens.
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Factors Affecting Acid Strength
Resonance in the conjugate bases of carboxylic acids stabilizes the base and makes the conjugate acid more acidic.
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Lewis Acids
• Lewis acids are defined as electron-pair acceptors.
• Atoms with an empty valence orbital can be Lewis acids.
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Lewis Bases
• Lewis bases are defined as electron-pair donors.• Anything that could be a Brønsted–Lowry base is
a Lewis base.• Lewis bases can interact with things other than
protons, however.
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• Soft covered book Pg. 288• Example and practice